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80 lines
3.9 KiB
Plaintext
80 lines
3.9 KiB
Plaintext
caption: Macro Definitions
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created: 20150220181617000
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modified: 20180820165115455
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tags: WikiText
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title: Macro Definitions in WikiText
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type: text/vnd.tiddlywiki
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A [[macro|Macros]] is defined using a `\define` [[pragma|Pragma]]. Like any pragma, this can only appear at the start of a tiddler.
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The first line of the definition specifies the macro name and any parameters. Each parameter has a name and, optionally, a default value that is used if no value is supplied on a particular call to the macro. The lines that follow contain the text of the macro text (i.e. the snippet represented by the macro name), until `\end` appears on a line by itself:
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<$codeblock code={{$:/editions/tw5.com/macro-examples/say-hi}}/>
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Alternatively, the entire definition can be presented on a single line without an `\end` marker:
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```
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\define sayhi(name:"Bugs Bunny") Hi, I'm $name$.
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```
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A more formal [[presentation|Macro Definition Syntax]] of this syntax is also available.
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!! Accessing variables and parameters
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Inside the macro there are several methods for accessing variables defined outside of the macro or parameters from the macro parameter list.
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|syntax|description|h
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|`$...$`|Text substitution of a parameter defined in the macro parameters list |
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|`<<__...__>>`|Parameter-as-variable access to a parameter defined in the macro parameters list |
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|`$(...)$`|Text substitution of a variable defined outside of the macro |
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|`<<...>>`|Access to a variable (or other macro) defined outside of the macro |
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<br>
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!!! Placeholders `$(...)$`
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The macro can contain placeholders for parameters. These consist of a parameter name between dollar signs, like `$this$`.
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The macro can also contain placeholders for [[variables|Variables]]. These consist of a variable name (or macro name) between dollar signs and round brackets, like `$(this)$`.
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The actual value of the parameter or variable is substituted for the placeholder whenever the macro is called:
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<$importvariables filter="$:/editions/tw5.com/macro-examples/say-hi-using-variables">
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<$codeblock code={{$:/editions/tw5.com/macro-examples/say-hi-using-variables}}/>
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<$macrocall $name=".example" n="1"
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eg="""<$set name="address" value="Rabbit Hole Hill">
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<<say-hi-using-variables>>
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</$set>"""/>
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</$importvariables>
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!!! Parameters as Variables `<<__...__>>`
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Parameters in a wikitext macro can be accessed as variables by using the syntax `<<__...__>>`, i.e the parameter name surrounded by double underscores. For example, the example above could also be expressed as:
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```
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\define sayhi(name:"Bugs Bunny") Hi, I'm <$text text=<<__name__>>/>.
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```
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Accessing parameters as variables only works in macros that are wikified and not, for example, when a macro is used as an attribute value. The advantage of the technique is that it avoids the parameter value being substituted into the macro as a literal string, which in turn can help avoid issues with parameters that contain quotes.
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For example, consider this macro. It invokes another macro using the single parameter as an argument for it:
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```
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\define film-quote(line) <$macrocall $name="anothermacro" actor="Bugs Bunny" line="""$line$"""/>
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```
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The code above will fail if the macro is invoked with the argument containing triple double quotes (for example `<<film-quote 'I quote thrice """ - see!?'>>`). Using parameter variables offers a workaround:
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```
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\define film-quote(line) <$macrocall $name="anothermacro" actor="Bugs Bunny" line=<<__line__>>/>
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```
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!! Scope
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Macros are available to the tiddler that defines them, plus any tiddlers that it transcludes.
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To make a macro available to all tiddlers, define it in a tiddler that has the tag <<.tag $:/tags/Macro>>.
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It is also possible to write a macro as a [[JavaScript module|https://tiddlywiki.com/dev/index.html#JavaScript%20Macros]]. ~JavaScript macros are available to all tiddlers, and offer the maximum flexibility.
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A tiddler can manually import macro definitions from a [[selection|Title Selection]] of other tiddlers by using the <<.wlink ImportVariablesWidget>> widget.
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